Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to devices for detecting the presence of an object, and more particularly to inductance type proximity sensors.
2. Description of the Related Art
Proximity sensors are commonly used along assembly lines to detect the presence of a work piece passing nearby. The presence of a work piece activates equipment that perform manufacturing operations.
One common type of sensor uses a transducer coil along with a tuning capacitor to form a resonant circuit of a free running oscillator. The transducer coil is located adjacent the path along with the work pieces pass. Eddy currents form in a conductive work piece approaching the coil and have a magnitude that is proportional to the magnetic flux lines impinging the work piece surface. The eddy currents alter the inductance of the transducer coil and the series resistance of the coil and its core. The change in the impedance affects the quality factor Q of the tuned circuit causing the peak-to-peak oscillator voltage to decrease in proportion to the change of the Q. Typically the loading of the coil by a work piece of ferrous metal causes circuit to stop oscillating. Thus the presence of a metallic object can be determined by monitoring whether the circuit is oscillating.
This type of proximity sensor has several drawbacks. In many applications, it is often desirable to know the relative position of the object being sensed. Such information is used to determine whether the object is properly positioned along the conveyor mechanism of the assembly line and coordinate the operation of manufacturing equipment. However, the sensor is binary in that its oscillator runs when an metallic object is not present and stops whenever a metallic object passes anywhere within the sensing range and can not provide information regarding the distance between the sensor and the work piece.
A second drawback relates to the detection of objects of different metals. Quite commonly, the sensing range of an inductance type proximity sensor is normalized using ferrous targets. As a consequence non-ferrous objects, such as those of aluminum, have an equivalent sensing range of approximately 45 percent of that normalized range. Thus, the sensor may not be capable of detecting non-ferrous work pieced passing along the far side of the assembly line from the coil. Conversely, if the proximity sensor is configured with greater sensitivity for non-ferrous metal objects, a ferrous object moving beyond the assembly line could trigger a false presence detection.
In some applications of these proximity sensors it is desirable to distinguish between ferrous and non-ferrous metallic objects. However present techniques for that discrimination are quite elaborate and often require expensive manufacturing processes or special metal disks and plastic spacers.
A proximity sensor for detecting presence of a metallic object includes a drive circuit connected to a transducer coil to generate an oscillating signal. That signal has oscillations which vary in response to whether or not a metallic object is adjacent to the transducer coil and in response to the distance between the object and the transducer coil. A processing circuit detects a characteristic of the oscillations and employs that characteristic to produce a indication of the presence of an object. Preferred embodiment of the proximity sensor employ the frequency and the decay rate of the oscillations to determine the presence of an object.
In one version the proximity sensor utilizes a resonant circuit that is stimulated by a voltage pulse of a short duration, after which the resonant circuit is allowed to ring down whereby the oscillations decay exponentially. The exponential decay of the signal in the resonant circuit is proportional to the quality factor Q, which varies in relation to the distance to a metallic object. This decay is characterized in a manner which enables a determination of not only the presence of a metallic object, but also the distance between the sensor and that object. Further analysis can determine whether the target is a ferrous or non-ferrous metal. As a result the sensor can be configured to respond to only ferrous or only non-ferrous objects.
Other aspects of the present invention include programming the circuitry of the proximity sensor by coupling a serial data signal to the transducer coil and providing a damping circuit to prevent the resonant circuit from ringing in response to the serial data signal.